Discussion
African swine fever is no longer an exotic disease and has established
self-sustaining, complicated transmission cycles in European wild boar
populations. Slow but constant local spread is observed (data from the
Animal Disease Notification System, visited online May
8th, 2020). This was rather unexpected as historical
experience did not indicate that wild boar could sustain an endemic
infection cycle (Laddomada et al., 1994). Field observations and
experimental studies indicate a high lethality (Blome et al., 2012,
Gabriel et al., 2011) and low contagiosity, especially in the initial
phase of an ASF outbreak among wild boar. The low level of contagiosity
requires a rethinking and an adapted approach to control ASF in the wild
boar population (Depner et al., 2016a, Depner et al., 2016b). Evidence
suggests that ASF in a feral pig population tends to behave more like a
long-term (rather stationary) habitat-bound disease with no tendency to
spread rapidly. It is mainly infectious cadavers, combined with the high
tenacity of the ASF virus and the low contagiosity, that can contain the
disease within a region (Depner et al., 2016b). ASFV-contaminated soil
rooted by wild boar is one of the habitat factors that could play a role
in transmission. Probst et al. (2017) reported that wild boar show
interest in the soil where carcasses have been found previously, with
wildlife cameras documenting animals rooting in soil even when only
bones remained. Furthermore, Estonian colleagues and others have
demonstrated viral genome in these soils (Viltrop and Nurmoja, personal
communication)(Zani et al., 2020).
In our study we tried to create a data set for a risk assessment of the
role of contaminated soil in ASFV transmission and possible mitigation
measures.
We demonstrate that virus stability depends on the soil type, pH,
organic material percentage, and to a lesser extent, on ambient
temperature. While contaminated sand retains infectivity for weeks,
virus stability is very low in acidic forest soils. Intermediate times
were found in swamp mud and yard soil. Within the limits of our
experimental setup, and assuming that the animal is an even more
sensitive detection system, we cannot rule out a persistence of
infectivity for at least a couple of weeks. The residual infectivity was
within the range that was shown to be infectious when orally applied to
susceptible animals (McVicar, 1984, Pietschmann et al., 2015). These
results contradict to a certain extent previously published studies
(Mazur-Panasiuk and Wozniakowski, 2020), where water, soil and leaf
litter inactivated ASFV quickly. In this study Mazur-Panasiuk and
Wozniakowski (2020) were able to re-isolate ASFV from soil and leaf
litter immediately after adding culture supernatant to the matrix, but
even a short 3-day incubation caused complete loss of virus infectivity
independent of temperature conditions. This is in line with our results
from swamp mud but not from yard soil or sand, where much longer periods
of infectivity were observed. In contrast, re-isolation immediately
after adding the contaminant to forest soil was impossible in our hands.
Thus, virus inactivation seems to occur after short contact with the
matrix e.g. due to the acidic conditions in both forest soil specimens
(pH 4.1 and 3.2).
Risk mitigation could involve the use of disinfectants despite the
obvious limitation that decontamination of soils in fields and forests,
which are very different in structure, consistency and composition, is
generally difficult and the organic matter in body fluids impairs
disinfection (Weber et al., 1999). We used citric acid and calcium
hydroxide in our study, which both have proven efficiency against ASFV
(Turner and Williams, 1999, Krug et al., 2012), the former with known
inhibition by blood (Krug et al., 2018). It must be noted that in the
past, lime products were used in the control of classical swine fever in
wild boar, e.g. in Germany. It is assumed that they not only have a
disinfectant effect but also repel wild boar. Furthermore, these
products were well accepted by the hunters. The application of lime was
therefore included in the official recommendation of the German
government for the use of disinfectants in an epizootic (Blome et al.,
2020). Despite the above information, it can be questioned whether the
application of a basic chemical to acidic soils in the wild boar habitat
is appropriate. ASFV is quite reliably inactivated at a pH of below 4
(EFSA, , 2009). Therefore, acidic disinfectants could be more useful and
here, citric acid was our candidate.
In our study, ASFV was inactivated after 1 h of disinfectant treatment.
In spiked beach sand and commercial potting soil not treated with
disinfectant, ASFV was fully inactivated after 2 weeks. However,
untreated blood or sterile sand were infectious for the entire test
interval of 3 weeks with consistent results from virus isolation
(macrophages) and titration on WSL cells.
In conclusion, ASFV stability is very low in forest soils but rather
high in sandy soils. Given the high variability of wild boar habitats
and unforeseen effects of the decay matrix, treatment of carcass
locations with disinfectants should be considered when setting up
control measures. The powder format of the used chemicals could be
beneficial and practical. Nevertheless, regulations on the use of
biocides and occupational safety have to be considered. Off-label use of
commercial products could be an alternative. In this context,
disinfectants based on potassium peroxymonosulfate (Trifectant, Virkon
S) were recently shown to inactivate ASFV on porous surfaces (Gabbert et
al., 2020) but had problems with blood under certain circumstances (Krug
et al., 2018). Removal of ASFV-positive carcasses is of utmost
importance and remains a critical control measure as live virus may
remain infectious in certain soil matrices for weeks. These studies
establish useful protocols to isolate ASFV from soil matrices, while
providing insight to potential management options useful in the field to
mitigate transmission.